Kinetic control of order in the layered perovskite La 2 Ba 2 Cu 2 Ti 2 O 11
- PDF / 248,561 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 59 Downloads / 209 Views
Welcome
MATERIALS RESEARCH
Comments
Help
Kinetic control of order in the layered perovskite La2 Ba2 Cu2 Ti2 O11 Maria Rosa Palac´ın and Pedro G´omez-Romeroa) Institut de Ci`encia de Materials de Barcelona (CSIC), Campus UAB, E-08193 Bellaterra, Barcelona, Spain
Frank Krumeich Institut f¨ur Angewandte Chemie, Berlin-Adlershof, Rudower Chaussee 5, D-12484 Berlin, Germany (Received 2 July 1996; accepted 28 January 1998)
Definitive evidence on the variable degrees of order/disorder in the layered perovskite La2 Ba2 Cu2 Ti2 O11 is presented. This oxide was prepared by the ceramic method with varying reaction times and cooling rates; the average and local structures of the resulting solids were studied by x-ray diffraction, electron diffraction, and transmission electron microscopy (low and high resolution). The data clearly show the presence of an ap 3 ap 3 4ap layered perovskite superstructure with a high concentration of defects in samples prepared over 48 h but with increasing degree of order for samples with longer reaction times. For samples reacted over 17 days, the order is complete and identical to that found for other related derivatives such as Ln2 Ba2 Cu2 Ti2 O11 (Ln Nd, Eu, Tb). This behavior clearly indicates the existence of a kinetic control of order in this phase.
I. INTRODUCTION 0
0
Complex perovskites of general formula AA BB O62y present a remarkable structural variability. Beginning with the ideal cubic perovskite structure (A2 B2 O6 ), we can first consider the possible distortions that take place as a consequence of unmatched sizes of A and B ions as described by the tolerance factor. In addition to these well-known distortions,1 the introduction of cationic complexity and especially the introduction of oxygen vacancies contribute to add a substantial structural variability to these systems.2–4 Understanding this variability is a first step for the control of order and dimensionality in the perovskite structure and for the design of novel phases with desired structure and properties. In the particular case of cuprate superconductors, one broad line of work has centered efforts in the development of bidimensional arrays of copper ions within the framework of the perovskite structure. In this sense we have conducted an extensive research on the relationship between synthesis structure and properties of complex perovskites containing copper and titanium.5–10 As a result of that work we isolated a whole series of layered perovskites of formula Ln2 Ba2 Cu2 Ti2 O11 with Ln La, Nd, Eu,7,8,10 and Ln Tb.9,10 The structure of this series presents an alternation of double copper and titanium layers derived from a cooperative order of oxygen vacancies, CuyTi and LnyBa pairs (Fig. 1). The only precedent of this novel type of perovskite superstructure is the previously reported a)
Author to whom correspondence should be addressed. J. Mater. Res., Vol. 13, No. 5, May 1998
http://journals.cambridge.org
Downloaded: 24 Mar 2015
complex perovskite La2 Ba2 Cu2 Sn2 O11 ,11 with tin instead of titaniu
Data Loading...
